Vinylidene fluoride (VDF) copolymers comprising recurring units derived from trifluoroethylene (TrFE) have been used extensively in the electronics packaging market due to their ease of processing, chemical inertness and attractive ferroelectric, piezoelectric, pyroelectric and dielectric properties.
As is well known, the term piezoelectric means the ability of a material to exchange electrical for mechanical energy and vice versa and the electromechanical response is believed to be essentially associated with dimensional changes during deformation or pressure oscillation. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
Ferroelectricity is the property of a material whereby this latter exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field.
Pyroelectricity is the ability of certain materials to generate an electrical potential upon heating or cooling. Actually, as a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence an electrical potential is established.
It is generally understood that piezo-, pyro-, ferro-electricity in copolymers of VDF with TrFE is related to a particular crystalline habit, so called beta-phase, wherein hydrogen and fluorine atoms are arranged to give maximum dipole moment per unit cell.
Copolymers comprising recurring units derived from vinylidene fluoride (VDF) and trifluoroethylene (TrFE) are typically provided as semi-crystalline copolymers which can be shaped or formed into semi-crystalline, essentially unoriented and unstretched, thermoplastic films or sheets or tubular-constructed products via well-known processing methods such as extrusion, injection moulding, compression moulding and solvent casting.
Nevertheless, more recently, developments of thin film electronic devices and/or assemblies of layers of ferroelectric polymers in three-dimensional arrays for increasing e.g. memory density have called for different processing techniques, requiring notably ability of the polymer to be patterned according to lithographic techniques and/or for layers there from to be stacked with annealing treatment on newly formed layer not affecting previously deposited layers.
Within this scenario, crosslinking (elsewhere referred to as “curing”), which is one of the most known techniques in polymer science to stabilize shape and fix structures, has been the technique of choice for accessing these needs.
Solutions have thus been proposed for conferring to VDF-TrFE copolymers cross-linking or curing ability. For instance, WO 2013/087500 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.) 20 Jun. 2013 discloses semi-crystalline VDF-TrFE fluoropolymers, further comprising recurring units derived from monomers comprising azide groups, which can be easily crosslinked either by thermal treatment or under UV irradiation while retaining inherent piezoelectric, ferroelectric, pyroelectric and dielectric properties.
Also, U.S. Pat. No. 6,680,357 (ATOFINA CHEMICALS INC.) 20 Jan. 2004 discloses acrylic-modified VDF-based fluoropolymers wherein the acrylic phase is capable of entering into crosslinking reactions. Nevertheless, these acrylic-modified VDF-based fluoropolymers are prepared by seed polymerization using the VDF-based fluoropolymer as a seed in the polymerization of one or more (meth)acrylic monomers.
There is thus still a need in the art for VDF-TrFE copolymer materials which can efficiently undergo crosslinking under thermal or UV exposure conditions, yielding uniformly cured materials which still maintain outstanding piezoelectric, ferroelectric, pyroelectric and dielectric properties.